1. Field of the Invention
The invention relates to optical fibers and to optical fiber sensors. The invention also relates to fabrication methods for optical fibers and to coupling methods.
2. Background Information
Fiber-optic technologies offering both sensing and signal transmission functions have attracted considerable attention in recent years, especially in smart concrete structures including highways, bridges, dams and buildings. Also, since optical fiber Bragg grating (FBG) sensors are of small size and light weight compared to textile yarns, and are readily embedded or ever woven inside textiles, they are the most promising sensor for smart textiles.
Optical fiber Bragg grating sensor comprises an optical fiber with a grating formed transversely across it by exposing the fiber to UV irradiation. The grating produces a differing refractive index within the core of the optical fiber. When light wave propagates along the core, part of the spectrum is reflected by the grating. The reflected wavelength (λ) is known as the Bragg wavelength (λB). The Bragg wavelength varies with events and conditions that the optical fiber is exposed to. In particular, the Bragg wavelength will vary with changes in temperature (T) and when the optical fiber is subjected to some form of strain (S). The equation for this change in Bragg wavelength is
            Δ      ⁢                          ⁢              λ        B                    λ      B        =                    C        11            *              S        L              +                  C        12            *      Δ      ⁢                          ⁢      T      where the constants C11 and C12 are functions of Poisson's ratio
  (            S      T              S      L        )and strain-optic coefficient, and thermo-optic coefficient and thermal expansion coefficient respectively.
The sensor will react to two types of strain: compression of the fiber in the transverse direction (ST) and strain of the fiber in the longitudinal direction (SL).
One of the most significant limitations of FBG sensors is their dual sensitivity to both strain (S) and temperature (T). This leads to difficulty in the independent measurement of these two measurands. To overcome this problem two FBG sensors, having different grating types, are located in close proximity. This arrangement reveals two equations.
                                          Δ            ⁢                                                  ⁢                          λ              1                                            λ            1                          =                                            C              11                        *                          S              L                                +                                    C              12                        *            Δ            ⁢                                                  ⁢            T                                                                        Δ            ⁢                                                  ⁢                          λ              2                                            λ            2                          =                                            C              21                        *                          S              L                                +                                    C              22                        *            Δ            ⁢                                                  ⁢            T                              
The above equations can be solved simultaneously to reveal the individual measurands. However, care must be taken when placing the two FBG sensors to ensure that they are subjected to exactly the same strain and temperature conditions.